(1) Field of the Invention
The present invention pertains to a system and a method for driving a liquid crystal display, in particular to an impulse-type application of such system and method.
(2) Description of the Prior Art
Liquid crystal display's main advantages are easy to achieve high resolution and its slim size. Therefore, liquid crystal display is widely used in notebook computers. Because of constant developments in large size display, liquid crystal displays also become the main stream monitors in desktop computers.
Please refer to FIG. 1. FIG. 1 is a basic diagram of the liquid crystal display 10. Within the liquid crystal display 10, a liquid crystal panel 12 has a plurality of pixels 121. By applying different driving voltage to liquid crystal molecules of pixel 121, the tilt angle of liquid crystal molecules can be changed. The liquid crystal molecules have “light valve” function. By controlling the tilt angle of liquid crystal molecules, the light transmitted from a backlight 14, which is disposed underneath the liquid crystal panel 12, can be controlled. Each pixel 121 of the liquid crystal panel 12 has a specific predetermined penetration rate to light at a predetermined time interval, which is associated with specific gray levels for forming visual frames.
Typically, in computers (as mentioned as notebook computers and desktop computers in above), screen displays mainly display in static-state condition. The so called “static state” means that a pixel 121 display the same gray level when a previous frame refreshes to a current frame. For example, when computer is running word processing program, most of the pixel 121 within the screen frame mainly displaying in “static state”, the pixel 121 of the area of currently word typing is called “dynamic state”. The so called “dynamic state” means that a pixel 121 displays different gray level when a previous frame refreshes to a current frame. For instance, the television screen mainly displays in “dynamic state”, or when using computer to play movie clips or animation clips, the screen also mainly displays in “dynamic state”.
The displaying characteristic of liquid crystal display is more suited for static-state displaying. The video control of liquid crystal display 10 depends on applying specific electrical voltage to liquid crystal molecules. Typical liquid crystal driving voltage is shown in FIG. 2A. It maintains a specific voltage during a frame time. This kind of method for driving liquid crystal display is known as a “hold-type”. The liquid crystal display 10 maintaining the “hold-type” driving mode typically uses a storage capacitor to store the voltage value of driving voltage until the driving voltage of the next frame being input to the storage capacitor.
As illustrated in FIG. 2, under the condition of maintaining a constant voltage value during a frame time, human's visional brightness perception is directly proportional to the accumulating time. Therefore, the liquid crystal display 10 using the “hold-type” will suffer dragging screen image when displaying dynamic-state frames due to the persistence of human vision. Human vision can easily perceive the remaining gray level of the previous frame when observing the current frame.
Please refer to FIG. 2B, which illustrates driving voltage of “impulse-type”. Another application of image displaying driving method is called an “impulse-type”. The “impulse-type” is typically used in traditional television or Cathode Ray Tube (CRT) monitor, because the electron beam of television or CRT monitor scanning speed is usually high, relatively. Typically, only one impulse is applied during one frame time. The display brightness is controlled mainly by coordinating the strength of electron beam. The brightness perceiving by human vision is the accumulation of brightness during the frame time. If the brightness accumulation value is the same during the frame time as that of a hold-type pixel, human vision identifies it as the same gray level. This means if FIG. 2A and FIG. 2B has the same brightness accumulation value during the same frame time, human vision will perceive exactly the same gray level.
As shown in FIG. 2B, the brightness of the “impulse-type” occurs at the early stage of each frame time because of the high speed scanning of electron beam. Therefore, from frame to frame, previous mentioned dragging screen image phenomena rarely happens, even under the “dynamic state”. This is because each frame can be isolated under the “impulse-type”, the residual brightness of the previous frame has less influence to human vision while perceiving the current frame.
Impulse-type driving is used in traditional television or CRT monitor. Currently, liquid crystal display also uses “impulse-type” driving. But it is mainly used in liquid crystal television products, very less in liquid crystal monitors for desktop or notebook computers. Although liquid crystal display with “impulse-type” can improve its drawback on dynamic-state displaying, however, it suffers flashing side-effect when displaying static-state frames. Therefore, using it in application of computer monitor products remained difficult. The flashing side-effect also is a typical problem in traditional television or CRT monitor.
According to the previous mentioned drawbacks and limits of prior liquid crystal displays, the objectives and summary of present invention are described below.